Both are correct it is called v droop if you're already running at 1.5 volts I wouldn't push further than that for 24/7.
Try bumping your nb to 1.3 volts
and maybe down clock your ram depending on what the default frequency is.

I am new to this forum.
I too have a athlon II X4 635 and tried to oc with default voltage. Wanted to get 240*14.5 so I could have 1600 mhz for ram
This is what i could achieve:
Cpu: 3407mhz
vcore: 1.40v
NB:2350
HT:1880
ram: 783 at 1.64v timmings 9 9 9 24
I could not get the ram go lower in timmings than this.

With that cooler you should be able to hit 3.8 as long as your MB lets you. Lower the memory speed, lower the HT link multi and bump up your NB multi.

Thanks of the tips, I'll get on that soon. This is my first real OC because my old CPU was not great for it, and I only got the V8 a couple weeks ago. up until that point I had a stock heatsink, which incidentally was the last stock component I had to replace.
The multiplier of the 640 is locked at 15, so I know I can only change the voltage and FSB, but other than that I don't know how far I can go exactly without resetting BIOS a lot, and that's getting annoying.
Also, my motherboard was bought at the same time and so far performs very well, it has a bunch of features regarding performance, so I'd be surprised if it couldn't hit 3.8. Honestly, it's the V8's performance I'm concerned about.Edited by Katcilla - 1/13/11 at 4:33am